1. Field of the Invention
The present invention relates to a navigation apparatus for detecting a current location of a mobile body.
2. Description of the Related Art
One type of known car navigation apparatus uses a recording medium such as CD-ROM which has map data stored therein. The map data includes road data which represent locations (or points) along streets/roads on a map by numbers. The car navigation apparatus detects a current location of the car and a moving direction of the car by using various sensors when the car is operated (or moving). At the same time, the car navigation apparatus reads a group of map data covering a certain area, which includes the current location of the car, from the recording medium to show a map of the area around the car in a display unit. The current location and moving direction of the car are both indicated automatically in the displayed map.
In the past, the navigation apparatus relies upon two navigation schemes. One scheme is self-contained navigation and the other scheme is GPS (global positioning system) navigation. The self-contained navigation utilizes self-contained sensors (e.g., acceleration sensor and angular velocity sensor) integrated in the navigation apparatus and does not receive any external support or information. The acceleration sensor and angular velocity sensor are used to detect (measure) navigation parameters such as acceleration and angular velocity of the car. The GPS navigation utilizes a plurality of artificial satellites which send measurement data (position data) to the navigation apparatus. The self-contained navigation has advantages and disadvantages, and the GPS navigation has advantages and disadvantages. In recent times, therefore, hybrid navigation is generally used, which combines (compares) measurement data supplied from the self-contained navigation with measurement data supplied from the GPS navigation.
In order to improve detection accuracy of a current car location provided by the self-contained navigation or GPS navigation, a map matching technique is also employed. The map matching technique or process causes the detected car location to shift to the road of the map data (road corresponding to the map data) while the car is moving, if the detected car location deviates from the road of the map data. One example of such map matching process is disclosed in Japanese Patent Kokai No. 10-307037.
In the map matching process disclosed in Japanese Patent Kokai No. 10-307037, map data is used, which is stored beforehand in a storage device. The map data indicates a location of the car on a road for map matching. The map data includes a region, in which a current car location obtained by the self-contained navigation or GPS navigation is present, and read from the storage device. Then, the car location on the road of the map data, read from the storage device, is compared with the car location obtained by the self-contained or GPS navigation. However, the car location on the road indicated in the map data is sometimes inappropriate for a driver of the car since an actual location of the car on the real road is very different from the car location presented by the map data when the car is moving under certain running conditions and/or in a certain area of the road (e.g., it depends which floor of a five-story parking garage (tower) the car is heading for or which lane of a three-lane highway the car is taking). In addition, map data itself is often not available in the storage unit for a particular location or region on the earth. As a result, the current location of the car cannot always be determined with high accuracy. This problem is also true to other types of navigation apparatus designed for other mobile bodies, such as a human.